Answer 1: Just compliment based on their hardwork. This student is obviously talented (perhaps even genius) but he'll become a useless dropout if he doesn't use those talents and get better through hard work. Talent only allows you to master something faster, not cheat the process. So don't say "wow, you learn fast, your a natural/genius!" That's the kind of comments that makes him bigheaded and discourage the other students who think that only those who are talented at martial arts can succeed in Aikido. This is totally the opposite, so instead, encourage them like "great hustle guys, I can see you all pushed yourselves really hard." or "good show of spirit, you all improved a lot this class."

Encourage them as a group, and that will develop a dojo esprit de corps and comradery. Compliment and reward based on hardwork and general improvement in the context of their own development. Don't compare them, as that will cause them to compete among one another and develop rifts in relationships, which is disharmonous. Let them compete instead on who works the hardest and encourage them to push beyond their comfort zone and grow.

2. Be who you are as you teach. If you use humour well, it can make the students relax when they are too overly tense or serious. I like it when the sensei jokes around a bit. My style's founder, the legendary Shioda Gozo, was quite the comedian. He talked to and joked around with everyone. When he trained he was serious, but when he talked he was funny. Being funny and not taking training seriously are two very different things.